A cross-current model describes physiologic and inert gas exchange in avian lungs. Physiologic determinants of the efficacy of such gas exchange in the goose will be defined. Distributions of ventilation and perfusion will be determined with the Multiple Inert Gas Elimination Technique (MIGET). The morphological basis of such inhomogeneities will be investigated with electron microscopy of lungs rapidly frozen under various physiologic conditions. The size of the functional unit of gas exchange in birds will be described as the size of the pulmonary vessels obstructed by beads just large enough to produce changes in V/Q distributions determined by the MIGET. Factors affecting the distribution of ventilation will be examined with a helium wash-through on unidirectionally ventilated birds. Airway diffusion limitations will be evaluated by comparing washouts of pairs of infused gases with similar solubilities, but different molecular weights. Membrane diffusion limitations will be quantitated at various levels of inspired O2 tension as the difference between arterial PO2 predicted by a model of avian O2 and CO2 exchange, considering the degree of V/Q inequality measured by the MIGET, and the arterial PO2 actually measured. Assumptions about distributions of diffusing capacity and blood flow required in the model will be investigated morphometrically in rapidly frozen lungs.